1. Field of the Invention
The invention has been developed from a method of electrolytically polishing a workpiece comprised of a nickel-, cobalt-, or iron-based alloy.
2. Description of the Prior Art
In the electrolytic polishing of metals and alloys of Groups IV B, V B, VI B, and VIII of the periodic table of the elements, the electrolytes used in most cases are based on concentrated acids such as sulfuric, phosphoric, or nitric. The action of the acid is intended to preferentially remove surface roughness during the anodic treatment of the metal objects (i.e., electromachining). With proper adjustment of the operating conditions this roughness removal can lead to smoothing of the surface. Various theories have been developed to explain the electropolishing process. For the most part these theories deal with the physical properties of the electrolytes. For example, the viscosity of the electrolytes is an important factor in diffusion at the anode. Another factor is the conductivity, which affects the current-voltage (current vs. voltage) characteristic.
Electrolytic polishing of rust-free steels is known from, e.g., Ger. Pat. No. 938,402, Ger. Pat. No. 754,703, Ger. AS No. 1,913,570, and U.S. Pat. No. 3,751,352. The corresponding process for superalloys is described in, e.g., Ger. OS No. 26 54 484.
Electrolytic polishing is comprised chiefly of two processes, which need to be carried out simultaneously: smoothing of the surface, i.e. the preferential dissolution of macroscopic and microscopic surface roughness without grain attack in the depressions, and dissolution of the metal oxide layer formed on the surface. In treating alloys, there is a third aspect which is even more important, namely the need to uniformly dissolve all the components of the metal. It is more difficult to achieve uniform dissolution of all the metals in the alloy to the extent that the component metals are from different groups of the periodic system. In nickel-based alloys of very complex composition (e.g., superalloys such as the "Nimonics") an electrolytic polishing effect may be achieved by the ordinary methods, but the result is a dull, matte surface, due to uneven dissolution of the different component metals, and also possible concentration of the difficulty soluble components at the surface being treated. An electrolyte based on perchloric acid has provided good electropolishing results, but such an electrolyte is not industrially feasible, because, e.g. the operating temperature required is too low, and there is a hazard of later corrosive attack through decomposed chloride ions. In addition, with perchloric acid there is an explosion risk, and the useful life of the electrolytes is too short.
Therefore, improvements are needed in the conventional methods of electrolytic polishing.
The problem giving rise to the invention is to devise a method of electrolytically polishing a workpiece comprised a nickel, cobalt, or iron alloy, particularly a superalloy, which method yields clean, smooth workpiece surfaces and can be carried out without explosion hazards, under normal shop conditions, at room temperature and at ambient temperatures. The electrolyte employed should have a high useful life.